Controlled Removal of Organic Dyes from Aqueous Systems Using Porous Cross-Linked Conjugated Polyanilines

Porous organic materials, as a broad class of functional materials, offer a promising route for low-cost purification of contaminated wastewaters. We have synthesized a range of highly cross-linked conjugated porous polyanilines and optimized their porosity and water dispersibility by tuning reactant feed ratios, previously unreported in the synthesis of such networks. To demonstrate their ability to adsorb model dyes used in the textile industry, we exposed the networks to a range of cationic aromatic dyes, leading to absorption capacities of >100 mg/g, reported for the first time with respect to polyaniline networks. The versatility of the networks was further demonstrated by the preparation of gel composites, producing active gels for efficient and facile removal and recycling, ideal for real-world applications. Finally, chemical modifications of the networks were undertaken to target the removal of model anionic organic dye pollutants, showing the wide applicability of our approach.


■ INTRODUCTION
Access to clean water is recognized as a key global goal for sustainable development under the United Nations Sustainable Development 2021 manifesto. 1 Despite the fact that it is considered to be a basic human right, only one in three humans have access to safe drinking water globally, 2 disproportionately affecting the lives of the vulnerable. 1, 3 More than 80% of wastewater resulting from human activities is discharged into rivers, lakes, or the sea without any treatment to remove pollutants. 4 One class of these pollutants includes organic dyes, typically highly pigmented organic compounds used in a number of industries including textiles, leather, paper, cosmetics, and food. Over 100,000 different dyes have been synthesized, and more than 0.7 million tons are produced annually. 5,6 Any effluent discharged by the textile industry is a major polluter and source of dyes released into the environment, as dyes are lost to the environment at every step of the process, from manufacture (10−15%) to application (20−30%). 7,8 In this study, we investigate the removal of five aromatic organic compounds as model dye pollutants: Acid Blue 92 (AB), Methylene Blue (MB), Ethyl Orange (EO), Direct Blue 15 (DB), and Congo Red (CR) (Figure 1). The four anionic azobenzene dyes AB92, EO, DB, and CR contain sulfonic groups (1−4, respectively), making them water-soluble and charged once dissolved (i.e., rendering them hard to remove from aqueous solutions). 9 The cationic thiazine dye MB is used as both a dye and medication, though it is reported as harmful at high concentrations (>2 mg/kg). 10 Once these dyes enter water bodies and break down, they form toxic and carcinogenic products, including naphthalene and benzamine. They also reduce levels of dissolved oxygen and light penetration due to their colored nature. Lower levels of oxygen and light inhibit the ability of aquatic plants and algae to photosynthesize, which in turn disrupts the aquatic ecosystem and biodiversity. In terms of damage to human health, it has been reported that dyes are carcinogenic and mutagenic as well as are irritants, causing contact dermatitis. 11−13 It is therefore imperative to remove dyes from water supplies at source.
Methods of dye removal or degradation can be categorized into three main groups: chemical (e.g., oxidation and reduction methods, photochemical degradation, electrochemical degradation, coagulation, ionic separation, and neutralization), physical (adsorption, filtration, precipitation, and reverse osmosis), or biological (sorption or degradation by plants, enzymes, or microbes). 9 Of these methods, adsorption is the most effective and widely investigated technique for contaminant removal: it is facile to process and possesses high efficiency and high selectivity when compared with degradation techniques.
Currently, activated carbon is the primary adsorbent employed in industry (2280 K tons consumed annually). 14 However, these materials face challenges regarding recyclability and sustainability and are often disposed of in landfill. 15 While natural-based (e.g., polysaccharide) 16,17 alternatives have been well researched for contaminant absorption applications, synthetic porous organic framework materials (POFs) are largely unexplored in the field despite representing a class of materials well suited to purification of water via adsorption. They possess high surfaces areas as well as exhibiting tunability of molecular functionality, physical properties, and surfaceaqueous interactions. 18−20 Here, we present a class of POFs based on cross-linked polyaniline structures 21 (Scheme 1) for water purification by dye removal. The formed materials are not only able to remove a range of organic dyes from water, but post-synthesis     Table 1 for ratios and  Table S1 for specific feed amounts used). An example is given for PTPA-3, where tris(4-bromophenyl)amine (0.25 mmol, 120.5 mg), p-phenylenediamine (3 mmol, 324.4 mg), 1,4-dibromobenzene (2.25 mmol, 530.8 mg), Pd(dba)2 ((dba = dibenzylideneacetone) 0.22 mmol, 201.5 mg, 4 mol %), 2-dicyclohexylphosphino-2′,4′,6′triisopropylbiphenyl (XPhos, 238.4 mg, 0.50 mmol), and sodium tert-butoxide (NaOtBu, 384.4 mg, 4 mmol) were added to a Schlenk tube and placed under a nitrogen atmosphere (see Table S1 for the full table of reactant masses added). Anhydrous toluene (50 mL) was added, and the reaction mixture was heated under stirring to 110°C. After 48 h, TLC analyses were carried out indicating complete consumption of the starting tris(4-bromophenyl)amine. The reaction was cooled to room temperature, and solids were removed by centrifugation. The solids were then subjected to Soxhlet extraction in chloroform, ethanol, methanol, and MQ water (400 mL each for 24 h) and dried for 72 h in a vacuum oven to yield corresponding amine networks as black-blue powders (see Figure S1) with yields of 65− 80% (see Table 1).

Characterization and Measurements.
Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer Spectrum 100 spectrometer, with samples in powder form. Thermogravimetric analysis (TGA) was carried out on a TGA Q500 apparatus in a nitrogen atmosphere (flow rate, 30 mL/min) in the temperature range of 30−800°C (heating rate, 20°C/min). Scanning electron microscopic (SEM) images were obtained on a JEOL 5600LV. X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer (40 kV, 30 mA) using Cu K α radiation (2θ = 5−45°). Nitrogen adsorption/desorption measurements at 77.4 K were performed after degassing the samples on a Schlenk line for 24 h and then under high vacuum at 70°C for at least 20 h on a Quantachrome Quadrasorb SI-MP apparatus. The specific surface areas were calculated by applying the Brunauer−Emmett−Teller (BET) model to adsorption or desorption branches of the isotherms (N 2 at 77.4 K) using the QuadraWin 5.05 software package. Analyses of the isotherms by commercialized quenched solid density functional theory (QSDFT) 22 and Grand canonical Monte Carlo (GCMC) 23 methodologies were also undertaken using the QuadraWin 5.05 package. The pore size distribution (PSD) profiles of the PTPAs were calculated from the adsorption branch of the isotherms with the Grand canonical Monte Carlo (GCMC) approach. 1 H NMR experiments were performed in D 2 O using Varian VNMR 400 MHz NMR. Contact angle measurements were performed at a temperature of 20 ± 0.5°C using a drop shape analyzer, DSA100 (KRÜSS), with 10 μL of Milli-Q water (resistivity of 18.2 MΩ·cm) droplets. The image of the drop was recorded for 300 s in 1 s intervals, and at least two repeat measurements per sample were made. UV−vis−NIR spectroscopy measurements were carried out using a Shimadzu UV-2600 spectrometer fitted with an ISR-2600 integrating sphere attachment. Measurements were recorded in 10 mm path length quartz cuvettes.
Dye Adsorption/Desorption Method. CMP (20 mg) was dispersed in deionized water (pH 7) by sonicating for 5 min. The dispersion was then exposed to the dissolved dye (40 mL) at initial concentrations of 0.0130 mg/mL (EO), 0.0156 mg/mL (AB), 0.0241 mg/mL (DB), and 0.0177 mg/mL (CR). Varying concentrations were used to achieve an absorbance of less than 1 unit, within the linear Beer−Lambert law range. Concentration change was determined over time (24 h) following the decrease in absorbance of selected ƛ max values for EO (475 nm), AB (575 nm), DB (600 nm), and CR (497 nm). The decrease in ƛ max absorbance was correlated to the dye concentration using a concentration curve with an R 2 > 0.99.
For the generation of filter absorption units, a Millex syringe filter was used (pore size, 0.8 μm; diameter, 33 mm) and PTPA-15 was loaded via flowing a dispersion of 10 mg/mL in DI water through the tip. For pH variation studies/desorption of EO, the same procedures were employed (after dye adsorption onto 20 mg of CMP for desorption) with a change of pH (via addition of HCl (aq) or NaOH (aq) to the desired pH) and ƛ max values for EO of 500 nm for pH <4 (no shift in ƛ max values for alkaline pHs was observed; a decrease in ƛ max absorbance under acidic conditions was correlated to the dye concentration using a concentration curve with an R 2 > 0.99).

■ RESULTS AND DISCUSSION
Synthesis and Preliminary Characterization. The PTPA extended networks PTPA-3-50 and the linear PAni (Scheme 1 and Table 1) were synthesized using a Buchwald− Hartwig cross-coupling reaction previously reported. 17 The successful synthesis of cross-linked networks was confirmed using several techniques (see Figure 2a−d for example data for PTPA-15). FT-IR spectroscopy demonstrated the disappearance of the carbon−bromine bond stretching vibration at 1004 cm −1 associated with the starting material (Figure 2b and Figure S2). Furthermore, the appearance of the secondary amine stretching vibration at 1305 cm −1 also suggested successful synthesis. UV−vis solid-state analysis of the networks revealed absorbances at 342 and 660 nm, characteristic of the π−π* transition of benzenoid and quinoid rings, respectively ( Figure S3). 12 Interestingly, it was observed that the relative intensity of the quinoid-based transition, with respect to the benzenoid, decreased as the linker length increased. Thermal analysis of the networks PTPA and PTPA-3-50 revealed degradation temperatures in the range of 232− 270°C (Table S2 and Figure S4); no direct correlation between degradation temperature and linker length was observed, although a significant increase in thermal stability with respect to PAni (193°C) was observed. No thermal transitions were apparent at lower temperatures, as expected in highly cross-linked networks. XRD analysis of the networks revealed broad amorphous features centered at 2θ = 12.5°, similar to earlier characterization of these and related materials  Figure S5). It is noted that the synthesized PAni was in the emeraldine base (EB) state.
To assess the suitability of the networks as adsorbents, surfaces areas were determined by BET surface area analysis. Analysis revealed a sharp decrease in surface area when extending networks (see Table 1) and an increase in the overall half pore width toward the mesoporous region (see Figure S6). As the mesoporous content of the networks increased, the networks tended toward the extended structure of the linear PAni, with consequent more favorable water interactions (i.e., more hydrophilic). This trend was first observed qualitatively by simple water dispersion tests: the extended networks retained superior dispersibility with respect to PTPA ( Figure  S7). This trend was confirmed by contact angle measurements. The contact angle decreased from 133.3°for PTPA (hydrophobic) to a range of 63.5−83.7°(hydrophilic) for the extended networks PTPA 3−50 and PAni (see Figure 2a for PTPA-15 and Table 1 and Figure S8). 24,25 Interestingly, the extended networks of PTPA 3−50 do not follow a uniform trend of decreasing contact angle toward that of PAni (increase seen for PTPA-9 and PTPA-50). This data suggests that, while the overall decrease in contact angle may be due to polyaniline-like linkers, the variations of contact angle in the extended region are likely caused by variations in the topology of the surfaces (corroborated by pore size distributions, see Figure 2c, SEM analysis in Figure 2d, and dye uptake kinetic studies discussed later). 26,27 Dye Adsorption Analysis. EO was initially selected to assess the suitability of the polymeric networks as an organic dye adsorbent (from aqueous systems at pH 7, see the Experimental Section for details). It was found that the 15extension PTPA-15 performed as the most effective dye

) is CR, panel (c) is DB where (i) is before network addition and (ii) is after network addition and filtration, panel (d) is AB, and panel (e) is EO where (i) is before network addition and (ii)
is after network addition but before filtration (to demonstrate visually the impact of adsorption) over 1500 min (at 24°C, pH 7, for initial dye concentrations, see the Experimental Section).

ACS Applied Polymer Materials
pubs.acs.org/acsapm Article scavenger with over 75% of dye absorbed within 5 min at a rate of 0.041 mg/min when using 20 mg of the adsorbent (Figure 3 and Table 2 and Figures S10−S12). It is worth noting that the extensions performed significantly better than the parent PTPA and PAni materials, with adsorption rates of 0.002 and 0.017 mg/min, respectively. For PTPA, it is suggested that the lower adsorption rate is due to the poor dispersibility of the networks in the aqueous environment, leading to a lack of contact with the dye (see contact angle data in Table 1 and discussion above). For PAni, it is suggested that the lack of porosity does not allow successful contact and dye entrapment. This aspect was illustrated in other studies, where porosity and uptake of PAni systems were enhanced by composite formation with porous materials. 28 Subsequent organic dyes tested on the networks included the anionic dyes AB, CR, and DB and the cationic dye MB. For each of the anionic dyes (Figure 1), increased removal capabilities were demonstrated by the extended networks over PTPA and PAni, with PTPA-15 proving to be the most efficient with respect to dye adsorption over time as well as the range of dyes absorbed (all anionic dyes removed within 1400 min, monitored by UV−vis; Figure 4). The increased removal rate of both CR and EO with respect to DB and AB is attributed to the latter dyes' solubility being a factor of 10 greater than that of both the former. Furthermore, analysis of rate of absorption kinetics revealed good fits (R 2 > 0.8, see Table S3) for extensions 9, 15, and 50 within the pseudo-firstorder absorption model for all dye absorptions. 29 Interestingly, PTPA showed low fitting values across individual order models and PTPA-3 showed better fitting for pseudo-second-order absorptions, potentially due to the surface topography (see Table 1), implying that the absorption process was enhanced by chemisorption. 30 The maximum absorption observed (where the solutions were saturated with dyes until no more absorption was observed by UV−vis measurements) reported in Table 2 shows dramatically increased uptakes when compared to non-synthetic dye-absorbent systems 31,32 (e.g., CR Abs max of 1 mg/g for plant material-derived systems vs 105 mg/g for PTPA-15) and competitive uptakes compared to activated carbon black (CR Abs max of 100 mg/g, see Table S4 for full comparative figures). 33−36 It is worth noting that the removal of the cationic dye MB was not achieved with these amino-based motifs (see discussion below).
Model binding studies were undertaken to understand the nature of the interactions of the organic dyes with the PTPA networks. Here, dilutions of CR (other dyes were not studied as signal dependency on the concentration was too low) in the presence of diphenyl amine (as the linker model) were analyzed using 1 H NMR in DMSO-d 6 (to ensure full solvation of both the dye and linker model, see Figure S13). Eight dilutions were made with a total combined concentration of 8 mg/mL for each of the dye and linker (0:8, 1:7, 2:6, 3:5, 4:4, 5:3, 6:2, 7:1, and 8:0 of dye to linker, respectively). The studies revealed changes to lower chemical shifts of the amidic protons of the CR with increasing linker concentration (from 7.685 to 7.675 ppm), indicating an increase in hydrogen bonding. However, the limited changes observed in aromatic resonances indicate a lack of π−π stacking interactions between the dye and the linker, at least for this model system. These investigations further confirmed that the surface area and porosity are the main factors accounting for the difference between the dye adsorption properties of the extended PTPA networks and PAni. The better performance of PTPA-15 when compared with other extended networks is linked to the increased propensity for dispersion as demonstrated by contact angle analysis (see Figure S14).
Variation of Aqueous Conditions of Dye Adsorption Studies. PTPA-15 was selected as the most versatile of the adsorbent materials to assess the effect of aqueous conditions on dye adsorption capacities of the extended networks (see Figures 3 and 4), with the salt content, temperature, and pH varied. First, the adsorption of CR was undertaken in saline conditions (35 mg/mL) to mimic industry relevant conditions. It was found that the addition of sodium chloride increased dye uptake ( Figure S14), with no CR dye detectable by UV− vis investigation after 10 min (in comparison to 360 min with no salt added). Although salt addition is a reported way of precipitating CR from aqueous environments (due to enhanced π−π stacking and lowered solubility 37 ), the use of the network as an adsorbent compared with simple precipitation using salt allows for removal of dyes at a far superior rate (minutes vs weeks) under mimicked conditions. 38 Elevated temperature adsorption investigations of CR with PTPA-15 were also conducted to mimic conditions encountered in industry. Adsorption was monitored in a water bath set at 80°C, while all other conditions (including solvent volumes, dye, and network amounts) were kept constant. It was again found that elevated temperatures aided adsorption, with 100% of CR adsorbed within 10 min ( Figure S15). While the rapid increase in dye absorption suggests a change from a pseudo-first-order to a second-order mechanism (see Table  S3), it is likely that simple increased diffusion of both the dye and CMP leads to increased rates. 27,28 EO was then selected as a model dye to study the effect of pH on the adsorption capabilities (CR was incompatible with such studies as its solubility is greatly decreased by variations in pH). 39 It was found that very low or high pH (2 and 12) reduced the uptake capacity of the dye to the network. Moderate changes in pH toward the pK a of the dye (4.34) gave increased uptake capacity (especially pH 4; Figure 5). It is proposed that the quinoidal state of the protonated EO allows for a better interaction with the networks at pH 4 (see the solid-state UV−vis spectra of the networks in Figure S3,  (2), the protonations of both sulfonate and amine groups of the dye and network, respectively, cause less attraction resulting in disruption of absorption (72% adsorbed after 1400 min). At higher pH values (12), the dye is fully deprotonated, hence loses the ability to hydrogen bond with the extended network, and therefore is not adsorbed efficiently (61% only after 1400 min). The theorized interactions of EO and the polyanaline-like networks are represented in Figure S16. Modification of the Adsorbent Systems. Three additional modifications and investigations were undertaken to demonstrate the versatility of the extended networks as dyeabsorbing materials and included preparing blended composites, exploring the dependence on pH of the environment, and changing the molecular architecture and function to target specific dyes for removal.
First, entrapment of the extended networks within a hydrogel was undertaken to improve manipulation of the formed blends (with respect to the powder-like form of the extended networks) after dye absorption. This approach ensured that a single solid mass could be removed from the aqueous solution after absorbance, rather than filtration being necessary, as shown in Figure 6. Hydrogels were formed from the cross-linking of polyvinyl alcohol (PVA) with terephthalaldehyde (TPA) in the presence of 37% HCl (aq) . Gels were either prepared with a 10 wt % loading of extended PTPA-15 or as control with no PTPA-15. It is worth noting that higher wt % PTPA-15 loading caused a failure in cross-linking, leading to unstable gels. After formation, gels were washed with water until neutral pH was achieved.
Despite control gels placed in the EO solution not absorbing any dye (Figure 6b, (ii), no decrease in the EO concentration was detected by UV−vis), complete absorption of EO (0.0130 mg/mL) was achieved after 24 h by the prepared composite CMP-containing gels (Figure 6b, (iii)). The practical advantage of our approach here is the facile removal of the gel after dye removal without the need for any filtration equipment (Figure 6b, (iii)). To test for possible contamination of the systems or CMP degradation, the experiment was rerun in D 2 O and the supernatant of the fifth cycle was

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pubs.acs.org/acsapm Article analyzed using 1 H NMR analysis, revealing no impurities or degraded product ( Figure S17). In addition, to explore the ability to recycle the loaded CMPs or their gel composites, we explored the use of facile pH changes to release the absorbed EO dye. For the gel/PTPA-15 blend (Figure 6c,d), addition of 1 M HCl (aq) (to achieve pH 2) led to 100% EO release over 30 min (Figure 6c). The use of pH to control absorbance and release for the parent powdered PTPA-15 was furthermore demonstrated in real time (see Videos S1 and S2, respectively, in the Supporting Information). PTPA-15 powder was loaded onto a Millex syringe filter, and a 0.013 mg/mL EO stock solution was filtered through to show immediate absorption. The release of this absorbed EO dye was achieved in real time over 30 s by treatment with a HCl solution at pH = 2.
These experiments clearly showed the versatility of the PTPA-15 in either gel form (Figure 6d, pH treatment of the gel composite and recycled five times) or as a powdered insert (Videos S1 and S2) to absorb and release dyes (with comparable efficiencies to well-researched polysaccharide systems), 40 showing the significant potential of these materials for real-life applications.
Finally, structural modifications of the extended networks were undertaken to target the cationic dye MB (not absorbed by networks PTPA and PTPA 3−50). Only modifications of PTPA-3 (selected as the best performing network) were undertaken. Here, the extended network formed from Buchwald−Hartwig cross-coupling reactions with 3,5-dibromobenzoic acid (rather than 1,4-dibromo phenyl units, see Scheme S1 for a complete reaction pathway) gave the extended structure functionalized with carboxylic acid moieties throughout (PTPA-3-COOH). Successful synthesis was confirmed by UV−vis and FTIR spectroscopy and surface area analysis (as for the extended networks, see Figures S2−S5 and S18). Although the functionalized network showed a decrease in surface area (37 m 2 /g), increased water dispersibility was immediately visible ( Figure S7) and confirmed by contact angle measurements (69.1 ± 2.3°).
Successful removal of the MB dye from water after 700 min was confirmed visually (see Figure 7a, (iii)) and by UV−vis measurements (Figure 7b).
Interestingly, the functionalized PTPA-3-COOH was not able to absorb any of the investigated anionic dyes. However, the formation of the anionic network as well as successful removal of MB demonstrates the versatility of these networks in regard to their tunability and synthetic modification for targeted contaminant adsorption.

■ CONCLUSIONS
In the search for simple and low-cost solutions to produce safe and clean drinking water, the use of heteroatom-containing conjugated microporous polymers was explored. Although such materials are often used in gas-capture settings (especially CO 2 capture), we have demonstrated here the formation of extended PTPA cross-linked networks and their suitability as water purification motifs. Investigation into the networks' abilities to remove pollutant organic dyes from aqueous systems has enabled initial optimization of the systems, leading to attractive performance and complete removal of cationic dyes in less than 10 min under conditions similar to those found in the dying industry. Further modifications, including blending and composite formation and their recyclability, showed the promise of these materials and approach for simple, low-cost, real-life applications. In addition, we have also shown the versatility of our approach, where simple structural modifications could be introduced to remove anionic dyes in a targeted fashion. Future modifications could include the addition of specific metal-coordinating moieties throughout the framework structure, thus potentially enabling the removal of highly toxic heavy metals such as Hg, Cd, or As. We therefore envisage that these materials will find wide application in low-cost filtration units for use in the informal dye industry and for water purification in rural settings in developing nations with contaminated water sources.